[PATCH] video/fbdev: Fix a double free in hvfb_probe

In function hvfb_probe in hyperv_fb.c, it calls hvfb_getmem(hdev, info)
and return err when info->apertures is freed.

In the error1 label of hvfb_probe, info->apertures will be freed twice
by framebuffer_release(info).

My patch sets info->apertures to NULL after it was freed to avoid
double free.

Signed-off-by: Lv Yunlong <redacted>
---
 drivers/video/fbdev/hyperv_fb.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/video/fbdev/hyperv_fb.c b/drivers/video/fbdev/hyperv_fb.c
index c8b0ae676809..2fc9b507e73a 100644
--- a/drivers/video/fbdev/hyperv_fb.c
+++ b/drivers/video/fbdev/hyperv_fb.c

@@ -1032,6 +1032,7 @@ static int hvfb_getmem(struct hv_device *hdev, struct fb_info *info)
 		if (!pdev) {
 			pr_err("Unable to find PCI Hyper-V video\n");
 			kfree(info->apertures);
+			info->apertures = NULL;
 			return -ENODEV;
 		}
 

@@ -1130,6 +1131,7 @@ static int hvfb_getmem(struct hv_device *hdev, struct fb_info *info)
 		pci_dev_put(pdev);
 	}
 	kfree(info->apertures);
+	info->apertures = NULL;
 
 	return 0;
 

@@ -1142,6 +1144,7 @@ static int hvfb_getmem(struct hv_device *hdev, struct fb_info *info)
 	if (!gen2vm)
 		pci_dev_put(pdev);
 	kfree(info->apertures);
+	info->apertures = NULL;
 
 	return -ENOMEM;
 }

-- 
2.25.1

Thanks for your patch.

I would like to change the prefix to "video: hyperv_fb:" to be more
specific.

On Tue, Mar 23, 2021 at 12:33:50AM -0700, Lv Yunlong wrote:

In function hvfb_probe in hyperv_fb.c, it calls hvfb_getmem(hdev, info)
and return err when info->apertures is freed.

In the error1 label of hvfb_probe, info->apertures will be freed twice
by framebuffer_release(info).

I would say "freed for the second time" here. What you wrote reads to me
fraembuffer_release frees the buffer twice all by itself.

My patch sets info->apertures to NULL after it was freed to avoid
double free.

I think this approach works. I would like to give other people a chance
to comment though.

Fixes: 3a6fb6c4255c ("video: hyperv: hyperv_fb: Use physical memory for fb on HyperV Gen 1 VMs.")

From: Lv Yunlong <redacted> Sent: Tuesday, March 23, 2021 12:34 AM

While I think this works, a slightly better solution might be to remove
all calls to kfree(info->apertures) in hvfb_getmem(),  and just let
framebuffer_release() handle freeing the memory.  That's what is
done in other drivers that follow the fbdev pattern, and it's less
code overall.  

Michael

-----原始邮件-----
发件人: "Michael Kelley" [off-list ref]
发送时间: 2021-03-24 02:52:07 (星期三)
收件人: "Lv Yunlong" [off-list ref], "KY Srinivasan" [off-list ref], "Haiyang Zhang" [off-list ref], "Stephen Hemminger" [off-list ref], "wei.liu@kernel.org" [off-list ref]
抄送: "linux-hyperv@vger.kernel.org" [off-list ref], "dri-devel@lists.freedesktop.org" [off-list ref], "linux-fbdev@vger.kernel.org" [off-list ref], "linux-kernel@vger.kernel.org" [off-list ref]
主题: RE: [PATCH] video/fbdev: Fix a double free in hvfb_probe

From: Lv Yunlong <redacted> Sent: Tuesday, March 23, 2021 12:34 AM

quoted

In function hvfb_probe in hyperv_fb.c, it calls hvfb_getmem(hdev, info)
and return err when info->apertures is freed.

In the error1 label of hvfb_probe, info->apertures will be freed twice
by framebuffer_release(info).

My patch sets info->apertures to NULL after it was freed to avoid
double free.

Signed-off-by: Lv Yunlong <redacted>
---
 drivers/video/fbdev/hyperv_fb.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/video/fbdev/hyperv_fb.c b/drivers/video/fbdev/hyperv_fb.c
index c8b0ae676809..2fc9b507e73a 100644
--- a/drivers/video/fbdev/hyperv_fb.c
+++ b/drivers/video/fbdev/hyperv_fb.c

@@ -1032,6 +1032,7 @@ static int hvfb_getmem(struct hv_device *hdev, struct fb_info

*info)
 		if (!pdev) {
 			pr_err("Unable to find PCI Hyper-V video\n");
 			kfree(info->apertures);
+			info->apertures = NULL;
 			return -ENODEV;
 		}

@@ -1130,6 +1131,7 @@ static int hvfb_getmem(struct hv_device *hdev, struct fb_info

*info)
 		pci_dev_put(pdev);
 	}
 	kfree(info->apertures);
+	info->apertures = NULL;

 	return 0;

@@ -1142,6 +1144,7 @@ static int hvfb_getmem(struct hv_device *hdev, struct fb_info

*info)
 	if (!gen2vm)
 		pci_dev_put(pdev);
 	kfree(info->apertures);
+	info->apertures = NULL;

 	return -ENOMEM;
 }
--
2.25.1

While I think this works, a slightly better solution might be to remove
all calls to kfree(info->apertures) in hvfb_getmem(),  and just let
framebuffer_release() handle freeing the memory.  That's what is
done in other drivers that follow the fbdev pattern, and it's less
code overall.  

Michael

Ok, i agree with you. Remove all calls to kfree(info->apertures)
in hvfb_getmem() is a better solution.

I will subimt a PATCH v2 for you to review. Thanks.